Gasification Australia
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Gasification Processes
Whilst our current technology development focuses on the small-scale biomass gasifier, there are many gasification technologies that could potentially be part of the future energy industry. One such system, receiving far too little attention in Australia in our opinion, is that of Supercritical Water Gasification (SCWG). Although a really large-scale commercial system is still some way off, the technology has some exciting possibilities. University of Melbourne researcher Doki Yamaguchi is currently working on the SCWG process. The approach and its potential are summarized below. Introduction Supercritical water gasification (SCWG) is an innovative conversion method for fuel production. It employs water above its critical point (T = 374 °C and P = 22.1 MPa), namely supercritical water (SCW). Especially for very wet biomasses, (eg agricultural and animal waste, sewage sludge, waste from the food and beverage industries etc), SCWG is potentially a more efficient and elegant approach than conventional thermal or digestion processes for both energy production and waste treatment. Conventional thermal processes (pyrolysis, gasification and combustion) require a large energy input for drying the feedstock and this makes some approaches impractical and costly. Digestion proceeds slowly and produces unconverted sludge which can have disposal problems. Brief chemistry The SCWG reaction is often studied with model biomass compounds such as glucose and cellulose since these are constituents of real biomass feedstocks and have similar chemical formulae. The stoichiometry of SCWG reaction is expressed as: C6H12O6 + 6H2O → 6CO2 + 12H2 (Glucose) C6H10O5 + 7H2O → 6CO2 + 12H2 (Cellulose) Theoretically, 1 mole of feedstock reacts with 6 or 7 moles of water to produce 12 moles of hydrogen and 6 moles of carbon dioxide. Attractive features Separation In the SCWG process, water acts as a solvent in which organic compounds and gases are easily dissolved whereas inorganic materials like metals, salts and ash are not. This SCW property gives SCWG a rapid conversion and a high feedstock utilisation, because the reaction proceeds in a homogeneous phase (the fuel is “dissolved” in the water) and hence there are no mass transfer restrictions. Furthermore, SCWG offers an effective CO2 separation (a near-pure stream of high pressure CO2 can be separated out), without the requirement of advanced separation technologies. This allows a SCWG process to be coupled to emerging sequestration technologies for zero or even negative carbon emission status. Selectivity for fuel production Another interesting feature of SCWG is that it is capable of product selectivity. The desired product can be controlled by manipulating the operating conditions and by the addition of catalysts. Low temperature SCWG (200-400°C) favours the production of a methane-rich product stream which can be used as an alternative to fossil natural gas. High temperature SCWG (600 – 700°C) is suitable for the production of hydrogen as a feedstock for fuel cell technology. In this form of SCWG, a large amount of hydrogen can be produced because water participates in the reaction. Thus the SCWG process enables the on-demand production of various high-grade energy resources from low-grade wet fuels and offers a new technological pathway for a sustainable future energy supply as the demand and price of fossil fuels increases and carbon emissions become regulated. Water treatment SCWG also offers a potentially attractive option for water recycling. For areas with limited rainfall, sustainable water supply is an important issue. In the conventional wastewater treatment process, most of the treated sewage is discharged into sea. The current recycling rate is around 10 % although more recycling is targeted in the near future. Prior to the development of the gasification process (SCWG), a super-critical water oxidation (SCWO) process has been developed and commercialised for the treatment of wastewater. SCWO efficiently destroys more than 99.9% of organic compounds in water with a much faster treatment time than conventional wastewater cleanup. The wastewater processing potential of the gasification process (SCWG) is expected to be similar. Thus, the SCWG process can be applied as both an effective wastewater treatment strategy to meet increasingly strict policies for environmental protection and recycle precious water, and at the same time, produce a valuable energy resource in the form of hydrogen or methane gas. Suitable applications Hydrogen production from wet feedstocks. [Of particular interest is sewage and Victorian brown coal] Methane production from wet feedstocks Integration with a CO2 sequestration scheme Wastewater treatment Reference Doki Yamaguchi et al, 2006 'Supercritical water gasification of sewage sludge', (Ref: 526-040) the International Association of Science and Technology for Development (IASTED) International Conference on Energy and Power Systems (EPS 2006), 29-31 March 2006, Chiang Mai, Thailand, pp. 1-9. Abstract |
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